The present invention relates generally to power system protection techniques, and more particularly, power system protection techniques that are relatively easy and relatively inexpensive to implement.
A power system (or electrical network) is said to be operating under steady-state conditions when there exists a balance between generated and consumed active power for the system. Power systems operating under steady-state conditions typically operate at or very near their nominal frequency. In the case of power systems within the United States of America, the nominal frequency is equal to sixty cycles per second (or sixty hertz).
Under certain circumstances, a power system can be disturbed such that it no longer operates under steady-state conditions. In that regard, power systems are subjected to a wide range of small or larger disturbances during operating conditions. Small changes in loading conditions occur continually. The power system must adjust to these changing conditions and continue to operate satisfactorily and within the desired bounds of voltage and frequency. A power swing condition can be the result of a disturbance that causes the power system to be removed from its steady state operating condition. Such power swings are characterized by variations in the power flow for a power system. These variations occur when the internal voltages of system generators slip relative to each other. Power system faults, line switching, generator disconnection, and the loss or the application of large amounts of load are examples of system disturbances that can cause a power swing condition to occur in a power system. Upon the occurrence of a power swing condition, there exists an imbalance between generated and consumed active power for the system. In particular, upon the occurrence of a power swing condition, there is a sudden change of the electrical power demand for the system. On the other hand, the mechanical power input to the system generators remains relatively constant. As a result of the power swing condition, the system generator rotors may accelerate and oscillations in the rotor angles for the sytem generators may occur, which can translate into severe system disturbances.
Depending on the severity of the system disturbance(s) and the actions of the power system controls during a power swing, the system may remain stable and return to a new equilibrium state, having experienced what is referred to as a stable power swing. However, severe system disturbances can produce a large separation of system generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventually lead to a loss of synchronism between groups of system generators or between neighboring utility systems. This occurence is referred to as an unstable power swing.
Large power swings, whether stable or unstable, can cause undesirable results. In particular, large power swings can cause the impedance presented to a distance relay to fall within the operating characteristics of the relay, away from the pre-existing steady-state load condition, and cause the relay to actuate an undesired tripping of a system transmission line. The undesired operation of system relays during a power swing can aggravate further the power system disturbance and cause system instability, major power outages and/or power blackouts. This can cause an otherwise stable power swing to become an unstable power swing. It will therefore be understood that distance relays preferably should not operate during stable power swings to allow the power system to establish a new equilibrium state and return to a stable condition.
During an unstable power swing, two or more areas of a power system, or two or more interconnected networks, lose synchronism. Uncontrolled tripping of circuit breakers during an unstable power swing condition could cause equipment damage and pose a safety concern for utility personnel. Therefore, it is imperative that the asynchronous system areas be separated from each other quickly and automatically in order to avoid extensive equipment damage and shutdown of major portions of the power system. During an unstable power swing condition, a controlled tripping of certain power system elements is necessary in order to prevent equipment damage, widespread power outages, and to minimize the effects of the disturbance.
Ideally, the asynchronous areas should be separated in such locations as to maintain a load-generation balance in each of them. System separation does not always achieve the desired load-generation balance. In cases where the separated local area load is in excess of local area generation, some form of non-essential load shedding is necessary to avoid a complete blackout of the system area.
To protect the power system, distance relays have integrated numerous protection functions including power swing detection and responsive relay blocking functions and unstable power swing detection and responsive selective tripping or pole slipping functions. The main purpose of power swing detection and responsive relay blocking functions is to differentiate faults from power swings and block operation of distance or other relay elements during all power swing conditions (stable and unstable power swings). In other words, during a power swing, it is ordinarily desirable to prevent tripping of the power system elements.
Faults occurring during a power swing must however be detected and cleared with a high degree of selectivity and dependability. Therefore, in such situations, the utilized power swing detection and responsive relay blocking function should allow the distance relay elements to operate and clear any faults that occur in their zone of protection during a power swing condition.
Power swing blocking functions are designed to detect power swings, differentiate power swings from faults, and prevent distance relay elements from operating during power swing conditions. Power swing blocking functions prevent system elements from tripping at random and at undesired source voltage phase angle difference between system areas that are in the process of losing synchronism with each other.
Unstable power swing detection and responsive selective tripping functions are also available in distance relays. The main purpose of these functions is to detect an unstable power swing condition by differentiating between stable and unstable power swing conditions. Power system utilities designate certain points on their network as separation points allowing for separation of asynchronous system areas during unstable power swing conditions. During an unstable power swing condition and at the appropriate source voltage phase angle difference between asynchronous system areas, the unstable power swing detection and responsive selective tripping function initiates controlled tripping of appropriate breakers (or other system elements) at predetermined network locations, to uncouple asynchronous system areas quickly and in a controlled manner in order to maintain power system stability and service continuity. Distance relay elements prone to operate during unstable power swings should be inhibited from operating to prevent system separation from occurring at random and in locations other than preselected ones.
Power swing detection and responsive relay blocking elements conventionally monitor the rate of change of the positive sequence impedance to detect power swing conditions. The required settings for these elements can be difficult to calculate in many applications, particularly those where fast power swings can be expected. For these cases, extensive stability studies are required in order to determine the fastest rate of possible power swings.
Unstable power swing detection and responsive selective tripping functions also typically monitor the rate of change of the positive sequence impedance. The required settings for this function are also difficult to calculate and in most applications it is required to perform an extensive number of power system stability studies with different operating conditions. This is a costly exercise and one can never be certain that all possible scenarios and operating conditions were taken under consideration.
The difference in the rate of change of the impedance vector has been conventionally used to detect a stable or unstable power swing and block the operation of the appropriate distance protection elements before the impedance enters the protective relay operating characteristics because it is known that it takes a finite period of time for the torque angle of system generators to advance due to system inertias. In other words, the time rate of change of the impedance vector is slow during stable or unstable power swings, because it takes a finite period of time for the generator rotors to change position with respect to each other due to their large inertias. On the other hand, the time rate of change of the impedance vector is very fast during a system fault.
Actual implementation of measuring the impedance rate of change is normally performed though the use of two impedance measurement elements together with a timing device. If the measured impedance stays between the two impedance measurement elements for a predetermined time, then a power swing is detected and a relay blocking signal is generated to prevent operation of the appropriate distance relay elements.
These conventional protection functions are mostly based on measuring the positive-sequence impedance at a relay location. During normal system operating conditions, the measured impedance is the load impedance and its locus is away from the distance relay protection characteristics on the impedance plane well known by those skilled in the art. When a fault occurs, the measured impedance moves immediately from the load impedance location on the impedance plane to the location representative of that fault condition on the impedance plane. During a system fault, the rate of impedance change is primarily determined by the amount of signal filtering in the relay.
During a power swing, the measured impedance moves relatively slowly on the impedance plane. For a power swing, the rate of impedance change is determined by the slip frequency of an equivalent two-source system.
This difference of impedance rate of change during a fault and during a power swing is utilized in conventional power swing detection schemes to differentiate between a fault and a swing. Placing two concentric impedance characteristics, separated by impedance ΔZ, on the impedance plane and using a timer to time the duration of the impedance locus as it travels between the characteristics is one manner used to make the differentiation. In that regard, if the impedance measured crosses the concentric characteristics within a predetermined period of time, then the event is deemed to be a system fault event. Conversely, if the impedance does not cross the concentric characteristics within the predetermined period of time, then the event is deemed to be a power swing.
Different impedance characteristics have been designed for power swing detection. These characteristics (identified as inner Z element and outer Z element) include the double blinders illustrated in FIG. 1, polygons illustrated in FIG. 2, concentric circles illustrated in FIG. 3, and lens characteristics illustrated in FIG. 4.
There are a number of issues that must be addressed to apply and set the power swing detection functions. To guarantee that there is enough time to carry out blocking of the appropriate distance relay elements following detection of a power swing, the power swing detection and responsive relay blocking function inner impedance (z) element must be positioned on the impedance plane outside the position of the largest distance relay protection characteristic on the impedance plane. Also, the power swing detection and responsive relay blocking function outer impedance (z) element must be positioned on the impedance plane at a position away from the position of the load region on the impedance plane to prevent power swing detection and responsive relay blocking logic operation caused by heavy loads, which would incorrectly cause blocking of the line mho tripping elements. These relationships among the impedance (z) measurement elements are illustrated in FIG. 2, using concentric polygons as power swing detection elements.
Those skilled in the art appreciate that these requirements are difficult to achieve in some applications depending on the relative line and source impedance magnitudes. It can be difficult to set the inner and outer power swing detection impedance (z) elements, and in certain circumstances incorrect relay blocking could occur.
Another shortcoming of conventional power swing detection schemes that measure the rate of change of the impedance is the determination and setting of the separation between the inner and outer impedance (z) elements and the determination and setting of the time period to be used to differentiate a fault from a power swing. These settings are difficult to calculate and depending on the power system under consideration, it may be necessary to run extensive system stability studies in order to calculate these settings.
Compounding matters further, the rate of slip between two system generators is a function of the accelerating torque and system inertias. In general, the slip cannot be determined without performing system stability studies and analyzing the angular relationships of system generators as a function of time to estimate an average slip in degrees/sec or cycles/sec. While this approach may be appropriate for systems having a slip frequency that does not change as a function of time, in many power systems, the slip frequency increases considerably after the first slip cycle and on subsequent slip cycles. In those instances, a fixed impedance separation between the inner and outer impedance (z) elements and a fixed time period for detection of a power swing might not be suitable to provide a continuous blocking signal to the mho distance elements.
Still another shortcoming of conventional power swing detection techniques is that they are very difficult to implement in complex power systems because of the difficulty in obtaining the proper source impedance values required to establish the inner and outer impedance (z) elements and the time period settings. In such power systems, the source impedances vary constantly due to network changes, for example due to additions of new system generators and other system elements. The source impedances could also change drastically during a major disturbance and during system conditions when the blocking functions are desired. Very detailed and extensive power system stability studies must be carried out, taking into consideration all contingency conditions in order to find the most suitable settings for the detection of the power swing.
Yet another shortcoming of conventional power swing detection and responsive relay blocking and unstable power swing detection and responsive selective tripping functions is that those functions are often combined together in a single logic structure within relays. This approach of combining the functions can present conflicting setting requirements if it is desired to apply both functions at the same transmission line location.
In view of the foregoing, it is desirable to provide a power system protection technique designed to protect against power swings occurring within the system.
It is also desirable to provide such a protection technique that separates the power swing detection and responsive relay blocking function from the unstable power swing detection and responsive selective tripping function. This will eliminate user confusion in the application of these relay functions and at the same time remove the conflicting setting requirements if it is desired to apply both functions in the same relay at the same transmission line location.
It is further desirable to eliminate user settings and the need for stability studies for the power swing detection and responsive relay blocking function.
It is still further desirable to provide for a power swing detection and responsive relay blocking technique that is independent of network parameters.
It is also desirable to provide for a power swing detection and responsive relay blocking technique that can be used effectively with long heavily loaded transmission lines of the type that present problems when using conventional techniques.
It is also desirable to provide for a power swing detection and responsive relay blocking technique that can detect three-phase faults that may occur during power swings and allow the protective relays to issue a tripping command and isolate the faulted power system element.
It is also desirable to provide for a power swing detection and responsive relay blocking technique that can track a power swing irrespective of the location of the apparent impedance in the complex plane.
It is still further desirable to remove the need for stability studies and simplify the settings for the unstable power swing detection and responsive selective tripping function when it is desired to trip on-the-way-out (TOWO).
It is yet further desirable to provide an option for the user to perform the unstable power swing detection and responsive selective tripping function on-the-way-in (TOWI).
These and other benefits of the preferred form of the inventive subject matter will become apparent from the following description. It will be understood, however, that a system and method could still appropriate the inventive subject matter claimed herein without having each and every one of these benefits, including those gleaned from the following description. The appended claims, not the benefits, define the exclusive subject matter and should be construed to the fullest extent permitted by law, including the applicable range of equivalency. Any and all benefits are derived from the preferred forms of the inventive subject matter, not necessarily from it in general.